Solar BalanceA battery is an optional investment. It helps when your panels produce a surplus at one time and you need electricity later. It adds the most financial value when the electricity it delivers replaces expensive imports and the surplus would otherwise earn little.
Measure the mismatch first
Look at generation and household demand at matching times, preferably using interval data. If your home already uses nearly all solar output directly, little remains to charge a battery. If demand is mostly after sunset, storage may shift more energy—but only when there is enough daytime surplus.
Energy Saving Trust explains that storage can move solar electricity to later use, while noting that its savings may not justify its purchase cost. Battery storage guide
Include storage losses and lost export income
A battery does not return every kWh put into it. Round-trip efficiency is the ratio of discharged energy to charging energy. Use a whole-system efficiency consistent with your calculator’s energy boundary. SAM battery metrics
For solar-only charging at flat prices:
Extra annual benefit = solar kWh charged × (round-trip efficiency × import price − export price)
Example: an illustrative 400 kWh charged, 90% efficiency, import price of 0.30 and export price of 0.05 gives 88 currency units of extra annual benefit, before maintenance or replacement. These are example assumptions, not local prices or a forecast.
Buy capacity you can use
Capacity in kWh is not the same as annual throughput. Charge power, discharge power, usable state-of-charge range, weather and demand timing constrain use. Batteries also lose capacity over time. SAM storage constraints
Compare the battery’s extra installed cost with its extra cash flow separately from the panels. Backup capability, grid charging and time-of-use trading need their own equipment and tariff assessment; this simple solar-shifting comparison does not include them.